What You’ll Learn
Is It Safe to Mix Different Types of Heating Oil?
Yes, most blends are safe, like No. 2, ultra-low sulfur, Bioheat®, or even diesel in an emergency, but not all.
You should always let your provider handle the mixing for safety and system protection because they know which fuels can mix without issue and which to avoid.
In this blog:
It’s unexpectedly chilly out.
You check the gauge on your heating oil tank.
It’s feeling a little too low for comfort.
You think about the gas station a few miles away.
Could a few gallons of diesel work for now?
When your heating oil tank starts to run low—or you’re dealing with an unexpected cold snap—it’s natural to wonder if you can safely “top off” with whatever fuel is available.
Before you run to the gas station, let’s take a closer look at which heating oils and fuels are compatible, which are not, and what to do if you ever find yourself in a pinch.
Understanding the Types of Heating Oil
Before we talk about mixing, it helps to understand what’s actually in your tank—and how the different fuel types compare.
Standard No. 2 Heating Oil
This is the most common type of home heating oil used in Pennsylvania.
It’s a refined petroleum product similar to on-road diesel, with a slightly different formulation for use in furnaces and boilers. No. 2 oil burns hot and efficiently, making it ideal for cold winters.
Ultra-Low Sulfur Heating Oil (ULSHO)
As part of a nationwide effort to reduce emissions, many states now require ULSHO instead of traditional No. 2 oil.
It’s essentially the same fuel, but with sulfur content reduced by more than 99%, producing cleaner combustion, less soot, and fewer maintenance issues.
Bioheat® and Biodiesel Blends
Bioheat® is a mix of traditional heating oil and renewable biodiesel, made from soybean oil, recycled cooking oil, or animal fats.
These blends are labeled by percentage—like B5 (5% biodiesel), B20 (20%), and so on.
Diesel Fuel
Diesel fuel is a combustible liquid used for powering diesel engines. Diesel fuel is obtained from less volatile fractions of crude oil than are used in gasoline.
Diesel and No. 2 heating oil are nearly identical in composition.
When It’s Okay to Mix Heating Oils
In most cases, mixing compatible types of heating oil is perfectly fine. The keyword is compatible.
Mixing Standard No. 2 Oil and ULSHO
This combination is completely safe. Both fuels share the same chemical properties; ULSHO simply contains less sulfur. When blended, they’ll burn cleanly and evenly without affecting performance or system health.
In fact, as ULSHO becomes the new standard across the region, many tanks already contain a mix from previous deliveries.
Mixing No. 2 Oil (or ULSHO) with Bioheat®
Mixing Bioheat® with standard oil is also safe and common. Most suppliers transition customers gradually by introducing biodiesel blends into existing tanks.
If your tank currently has regular heating oil, adding Bioheat® won’t cause issues. The renewable component simply improves combustion quality and reduces emissions over time.
Using Diesel Fuel in a Pinch
If you’ve run out of heating oil and the temperature is dropping fast, diesel fuel can safely get you through the night.
It’s effectively the same product, just taxed differently and dyed clear instead of red.
You can pour diesel directly into your tank to keep your furnace running until your next delivery.
However, diesel costs more per gallon, and prolonged use can increase wear on your system, so it’s best as a temporary fix only.
When Mixing Fuels Is Not Safe
While most heating oil blends are compatible, there are a few fuels that should never be mixed or used in your heating system.
These fuels include:
- Gasoline
- Kerosene
- Unknown fuels
NOTE: If any of these have been added to your tank, shut off your system immediately and call a professional before attempting to start it again.
Gasoline
Never, ever add gasoline to your heating oil tank.
Gasoline has a much lower flash point (the temperature at which it ignites) than heating oil or diesel.
Mixing it with heating oil can cause
- Flare-ups
- Fires
- Explosions in your burner or tank
Kerosene (Unless Your System Is Designed for It)
Some older or mobile-home heating systems are designed to burn kerosene, but most residential oil furnaces are not.
Kerosene burns hotter and “drier,” which can damage seals, filters, and burner components over time.
If you’re unsure whether your system can handle kerosene, don’t risk it—call your fuel provider to check your unit’s compatibility.
Mixing Random or Unknown Fuels
Never pour in leftover or unidentified fuels, even if they look similar to heating oil.
Small differences in additives, viscosity, or contamination can cause system clogs, smoke, or damage to your pump and nozzles.
Why Mixing Should Be Left to Your Provider
Even when fuels are compatible, the process of mixing them properly and ensuring they meet your system’s specifications should be handled by a professional.
Here’s why:
- They know what’s already in your tank: Your provider can test or verify the current fuel type, ensuring compatibility before adding anything new.
- They can prevent contamination: Fuel storage tanks can accumulate water, sludge, or residue that react differently with new fuel types. Your provider can flush or filter the tank as needed.
- They can verify system readiness: A technician can check seals, filters, and burner settings to make sure your system runs smoothly after a mix or changeover.
In short, you shouldn’t play chemist with your heating oil tank. Mixing fuels may seem simple, but professional oversight protects your system and your safety.
What to Do If You’re Running Low on Heating Oil
If you realize you’re running low (or you’ve already run out), don’t panic. Here’s what you should do:
- Turn off your furnace or boiler: This prevents air from getting trapped in your fuel line, which can make restarting harder later.
- Call your fuel provider immediately: Many companies, including Sperr’s Fuel and Heating, offer same-day or emergency heating oil delivery to get you back up and running quickly.
- Use diesel only if absolutely necessary: If you can’t wait for delivery and temperatures are dangerously low, add a few gallons of diesel fuel from a gas station. It’ll safely operate your system for a short period.
- Schedule a refill and system check: Once your provider refills your tank, it’s a good idea to have them inspect your system to ensure there’s no air in the line or residue from mixing fuels.
If You’re Unsure, Call Sperr’s
At Sperr’s Fuel and Heating, we help homeowners across Pennsylvania stay warm safely and affordably.
Whether you’re running low, switching to a cleaner blend, or just unsure what’s in your tank, our team can confirm your fuel type, recommend the right next step, or even deliver same-day heating oil if you need it.
Contact Sperr’s today for expert advice, emergency delivery, or dependable heating oil service you can trust.
FAQs
Can I mix ultra-low sulfur heating oil with regular No. 2 oil?
Yes. Ultra-low sulfur heating oil (ULSHO) and No. 2 are fully compatible.
Mixing them won’t harm your system or affect performance—it simply reduces sulfur content and emissions.
Many homeowners already use a natural blend of both as suppliers phase out higher-sulfur fuels.
Can I use diesel fuel in my heating oil tank?
You can safely use diesel fuel as a temporary substitute for heating oil if you run out.
It’s chemically almost identical to No. 2 oil. However, diesel is more expensive and not ideal for long-term use, so schedule a delivery as soon as possible.
Is Bioheat® safe to mix with standard heating oil?
Absolutely. Bioheat® is a blend of biodiesel and traditional heating oil, designed for full compatibility with standard systems.
Mixing Bioheat® with regular heating oil simply results in a cleaner, renewable blend that reduces carbon emissions and keeps your furnace running efficiently.
What happens if I accidentally add the wrong fuel, like gasoline?
Adding gasoline is extremely dangerous. It ignites at much lower temperatures and can cause fires or explosions in your burner.
If this happens, turn off your heating system immediately and call your fuel provider or an emergency technician for assistance.
How can I tell what type of heating oil is in my tank?
Check your delivery receipts or tank label for details. If you’re unsure, your provider can test a sample to confirm.
Your provider can help you identify your current fuel, recommend the right blend, and ensure safe, efficient operation year-round.